The present invention relates to a method and a system for operating a brake system of a motor vehicle having traction control. The term xe2x80x9coperationxe2x80x9d here refers to both open- and closed-loop control.
In operating brake systems in motor vehicles, it is important to determine exactly the actual brake pressure that occurs in a vehicle. In particular, the brake pressure on the vehicle wheels is determined in order to operate the brake systems of vehicles, e.g., for ABS, ASR or ESP control systems.
It is known that the brake pressure and braking torque are determined by calculating the brake pressure from states between the road surface and the wheels on the vehicle as derived from wheel speed signals. For a long time, the forces acting between the wheel and road surface could not be determined directly for lack of suitable sensors. Instead, the signals required for control purposes were derived or calculated from wheel speed signals.
Meanwhile, sensors have become available to permit direct determination of the state prevailing between the road surface and the vehicle wheels.
For example, it is known from the article by Jxc3x6rg Stxc3x6cker et al. xe2x80x9cDer xe2x80x98Intelligent Reifenxe2x80x99xe2x80x94Zwischenergebnisse einer interdisziplinxc3xa4ren Forschungskooperationxe2x80x9d (xe2x80x9cThe xe2x80x98Intelligent Tirexe2x80x99xe2x80x94Interim Results of an Interdisciplinary Research Cooperationxe2x80x9d) in the automotive engineering journal ATZ Automobiltechnische Zeitschrift, vol. 97 (1995) no. 12, pp. 824-832 that a tire can be equipped with an integrated force sensor which makes it possible to detect forces acting on the tire in three directions, namely the longitudinal or X direction, the transverse or Y direction and the vertical or Z direction. The signals derived from the forces detected in this way can be sent to an analyzer unit where the signals are analyzed appropriately.
Another device for determining the rotational behavior of a vehicle wheel is described in German Published Patent Application No. 196 20 581, for example, which describes a device for determining the rotational behavior of a vehicle wheel, where magnetizing surfaces having alternating polarities are arranged uniformly in the circumferential direction of the wheel. The surfaces are incorporated into or applied to the tire wall. A measuring sensor has two or more measuring elements arranged at different radial distances from the axis of rotation, so that there is a change in phase angle between the signals delivered by the measuring elements in the case of deformation of a tire due to the forces acting on the tire or due to the driving torque or brake torque transmitted. The change in phase angle can then be analyzed as a measure of the torques transmitted from the wheel or tire to the road surface and/or the instantaneous coefficient of friction.
German Published Patent Application No. 196 12 825 describes a roller bearing or a wheel bearing which is designed to detect driving forces and/or braking forces.
With the method and/or the system according to the present invention, it is possible to use a sensor designed according to the principle described in the ATZ article mentioned above or to use a sensor based on the principle described in German Published Patent Application No. 196 12 825.
Although tire sensors have often been described in the related art as useful for controlling the driving behavior of motor vehicles, it has not previously been known that a brake system of a vehicle can be improved by using signals from sensors that measure the forces acting between the vehicle wheel and the road surface with a view to determining the brake pressure prevailing on each of the vehicle wheels.
Therefore, the object of the present invention is to create a method and a system with which the operation of a brake system of a vehicle having traction control can be simplified and improved.
In particular, the method according to the present invention and the respective system permit a simple determination of the precise brake pressure actually occurring in each vehicle. The pressure sensors that determine the brake pressure in a brake system, in particular in electrohydraulic brakes, are no longer necessary here, thus eliminating the need for complicated assembly, adjustment, monitoring and plausibility checks as required with the previous pressure sensors and eliminating the costs of the relatively expensive pressure sensors. This also eliminates the complicated A/D conversion required with the previous pressure sensors.
The pressure quantities determined for a vehicle wheel can be sent to a control unit and processed further in algorithms of ABS, ASR or ESP control systems. Since the pressure quantities determined describe the actual brake pressures occurring, the control can be improved significantly.
For example, the following parts in the ASR algorithm can be improved greatly: holding times in pressure buildup can be lengthened, because a pressure buildup can be initiated immediately when the precise brake pressure is known, and holding times in pressure reduction can be lengthened, because a blocking torque can be reduced accurately in defined stages when the precise brake pressure is known and thus better traction can be achieved without the wheel running up to the reference speed or running into brake slip. This method can be used with xcexc-split braking, for example. Gradient switching in a pressure buildup or reduction can be improved because through exact determination of the braking torque, it is possible to ensure that the switching induces the predefined effect with respect to wheel response.
Complicated hydraulic models which were used previously for determining the most accurate possible value for the actual brake pressure are also unnecessary. Faults occurring in the hydraulic system such as hydraulic leakage or jamming of a valve can be detected easily. Effects of these faults, which might be noticed by the driver of the vehicle in a negative manner, can be prevented easily by compensatory regulation.
In the method according to the present invention, a quantity for the respective vehicle wheel is detected using sensors assigned to the individual vehicle wheels, so that the quantity represents at least the driving forces and/or braking forces acting between the road surface and the vehicle wheel. In particular, pressure quantities describing the brake pressure for the respective vehicle wheel are determined as a function of the quantities detected with the sensors. The pressure quantities thus determined are then analyzed for operation of the brake system having traction control.
The quantities detected by the sensors advantageously result from the motion and/or rotation of the vehicle wheel, these quantities being output by the sensors as phase-modulated or amplitude-modulated signals.
In an analyzer unit, a quantity that describes the circumferential force of the respective vehicle wheel is determined from the quantities detected by the sensors and is then used to determine the brake pressure.
The sensors are provided on each vehicle wheel in particular, so that a pressure quantity is determined for each individual vehicle wheel, and these pressure quantities are sent to a control unit for regulating the brake pressure according to the pressure quantities thus determined.
The quantity describing the circumferential force of the respective wheel is advantageously determined using a characteristic curve stored in the analyzer unit as a function of the respective phase- or amplitude-modulated signal. The brake pressure can be determined from the circumferential force by multiplying it by a predetermined factor.
The use of a characteristic curve is explained in greater detail below.
Driving and/or braking forces act on the tires, causing them to be deformed. This deformation causes a displacement of the magnetic particles incorporated into the tires. This displacement results in modulation of the phase and/or amplitude of the signal delivered by the sensor. The intensity with which the amplitude and/or phase is modulated is a measure of the driving and/or braking forces acting on the tire. Consequently, the circumferential force Fu can be determined from this signal by using a characteristic curve stored in the analyzer unit. As an alternative to this, it is also conceivable to determine the circumferential torque. If we then consider the case of braking, then the tension force on the brake shoe that is to be applied to achieve the circumferential force can be determined by using the equation Fm=Fu/C*, where C* is a brake characteristic stored in the analyzer unit. The braking torque achieved in braking is obtained from the equation MB=Fmxc2x7r, i.e., the braking torque is the product of the frictional forces induced by the tension force multiplied by the distance of the points of application of these forces from the axis of rotation of the wheel. The brake pressure to be established for the respective wheel is obtained from the equation PB=MBxc2x7C**.
Taking into account the preceding equations, the following equation is thus obtained for determining the brake pressure:
PB=Fuxc2x7rxc2x7C**/C*=Fuxc2x7Cp.
In summary, this means that the quantity describing the circumferential force of the respective wheel is obtained by using a characteristic curve stored in the analyzer unit as a function of the phase- or amplitude-modulated signals, i.e., as a function of the deformation occurring on the tire. The brake pressure is determined from the circumferential force by multiplying it by a predetermined factor Cp.
In addition, the signals for a respective brake pressure are sent as actual values directly to the control unit for a brake, in particular an electrohydraulic brake.
The signals for a respective brake pressure for each individual vehicle wheel which are sent as actual values are processed further in the control unit in an algorithm for controlling the brake.
The signals for a prevailing brake pressure for each individual vehicle wheel which are processed further in an algorithm for controlling the brake are sent as control signals to valve controls for brakes for each individual vehicle wheel.
The newly determined signals for the prevailing brake pressure and/or a pressure quantity for each individual vehicle wheel is subjected to PT1 filtering after being entered into the control unit. A check is performed to determine whether a predetermined rate of pressure change, representing the maximum allowed pressure difference of two pressure quantities determined at successive times is exceeded and whether the signals are within predetermined pressure limits. On fulfilling the conditions, the newly detected signal is sent as an actual value to the valve control. New pressure quantities are determined for each individual vehicle wheel only after a predetermined period of time.
During a pressure holding phase of the valve control, the signals for a prevailing brake pressure or pressure quantities for each individual vehicle wheel which are determined as actual values are advantageously compared with setpoint values for the brake pressure. Then if the deviation is greater than a predetermined value, the conclusion is drawn that there is a hydraulic leak or a jammed valve. When a hydraulic leak is detected on a certain valve, a pressure buildup is initiated immediately in the algorithm part of the control unit and the valve control. In the case of jamming of a valve, a pressure reduction is initiated immediately.
In the system according to the present invention for carrying out the method according to the present invention, the control unit has in particular a processing unit which subjects signals to a PT1 filtering after they are entered and then determines whether a predetermined rate of pressure change, representing the maximum allowed pressure difference between two pressure quantities determined at successive times, is exceeded and whether those pressure quantities are within predetermined pressure limits. On fulfilling the conditions, the processing unit sends the pressure quantities thus determined as actual values to the valve control. The processing unit preferably has a timer for defining a predetermined period of time after which it repeats a determination of the actual values.
In addition, during a pressure holding phase of the valve control, the processing unit compares the pressure quantities determined as actual values for a prevailing brake pressure for each individual vehicle wheel with setpoint values for the brake pressure, and if the deviation is greater than a predetermined value, it draws the conclusion that there is a hydraulic leak or jamming of a valve.
The control unit and the valve control immediately initiate a pressure buildup in the event a hydraulic leak is detected, and they immediately initiate a pressure reduction in the event a jammed valve is detected.